1. Field of the Invention
The invention concerns an arrangement for controlled admission to the nozzle assembly of axial flow turbomachines, in particular extraction steam turbines, having a control valve which is located upstream of the nozzle assembly and has at least two inlet windows for the working medium.
2. Discussion of Background
In steam turbines, nozzle group control is particularly suitable for installations from which high part-load efficiencies are demanded. The first turbine stage, also referred to as the control stage, is usually equipped with impulse or Curtis blading and has several admission sectors, the steam flow from the steam generator to each of the admission sectors being adjusted by a special control valve. It is usual to open one control valve after the other in a continuous manner with increasing output of the steam turbine. At a given load condition, therefore, a larger or smaller number of control valves is in general fully opened and there is, therefore, no throttling. Only one of the control valves is partially opened and causes an additional throttling loss. This loss can be kept to a modest amount because it only affects the partial mass flow flowing through the relevant control valve and this partial quantity becomes smaller as the number of admission sectors becomes larger. It follows from this that an infinitely large number of control valves and admission sectors should, ideally, be provided. In practice, machines are even known which have up to 10 admission sectors and associated control valves and therefore permit very sensitive control. The most common arrangement, however, is one with 4 segments; almost all reheated turbines now operate in this manner. In the case of 4 valves, it is therefore possible to arrange a nozzle distribution of 20%, 20%, 30%, 30% around the periphery. By this means, it is possible to operate valve points when the machine is running with approximately the following powers: 30%, 60%, 90%, 100%. With the high steam temperatures now usual, the nozzle areas first opened are usually arranged symmetrically in the lower part and the upper part of the casing so that asymmetrical temperature distributions are avoided when starting up.
Arrangements for controlling the flow cross section of a turbomachine are also used in steam extraction turbines. They permit a variable mass flow of steam to be branched off--for process purposes, for example. In the case of conventional, axial flow steam turbines, such controlled extraction systems are known in which, after flowing through one turbine section, the whole of the mass flow is led out of the turbine, controlled and subsequently reintroduced into the following turbine section. For each internally controlled bleed a plurality of sequentially opening relief adjustment valves are flanged onto the turbine casing; these control the quantity of steam flowing into the subsequent turbine section and, by this means, keep the extraction pressure constant.
Also known are arrangements by means of which the free flow cross section in the nozzle assembly is modified to control the steam mass flow, likewise in the form of adjustable guide vanes. The guide vanes can, for example, be rotated about their own longitudinal axis in order to reduce the cross section. The center of rotation can be at the leading edge of the vane, within the vane profile or at the trailing edge of the vane. In all these variants, the flow cross section can be completely shut off during an adjustment. In addition, the vane geometry, which is important for aerodynamic reasons, is maintained. In these arrangements, however, the inlet flow to the guide vanes and the outlet flow from them is modified to a greater or lesser extent and this impairs the mode of operation of at least those rotor blades which immediately follow.
Arrangements of the type quoted at the beginning are known from the journal article "Zur Entwicklung von Niederdruck-Dampfsteuerorganen, derzeitiger Stand und zukunftige Moglichkeiten" (The development of low pressure steam control units, current status and future possibilities), Maschinenbautechnik, Berlin, 38 (1989), pages 17 ff. In a first variant, the working medium, low pressure steam in this case, enters an annular chamber upstream of the guide vane cascade via radial rotary valves with a large number of inlet windows which can be shut off. In a second variant, the working medium enters the guide blading directly via axial rotary valves with a large number of inlet windows which can be shut off. Both arrangements are suitable for throttling control, the rotary valves being displaced by one window pitch at a time from the fully open condition to the fully shut-off condition.